Photo-electric Coupling Effects In High-efficiency Multijunction Solar Cells And Optimization-designed Cells With Space-radiation Resistance

Multijunction solar cells are the core photoelectric components of the power system in spacecrafts.With using new materials?AlGaInP,InGaAs et al?,developing some new structures?flexible and metamorphic solar cells?and processes?metamorphic and quantum dots?in multi-junction cells,series of new phenomena have been detected,such as luminescence coupling and light modulation effects.Both could exert significantly effects on the performance of multijunction cells as well as measurement methodology.Meanwhile,multijuntion solar cells are promising and exposed to space radiation environment consisting of particles with specific spectra in specific orbits.The non-uniform radiation damages among or within sub-cells becomes an important bottleneck for evaluating the behaviors or optimizing the anti-radiation property of the multijunction solar cells.In this paper,some newly-designed multi-junction III-V solar cells and the corresponding sub-cells were selected to investigate on the modulation and luminescence coupling effects and their physical mechanisms,as well as the effects of particle irradiation damage using complementary advanced measurement methods,physical models of semiconductor devices and circuit models.Afterwards,the non-uniform damage behavior and mechanisms on the solar cells after particle irradiation were studied systematically.An equivalent evaluation method was established for multijunction solar cells applied in orbits with particle spectral radiations.Based on these theoretical results,one could design and optimize the multijunction solar cells with both high optical-electronic conversion efficiency and promising radiation-resistance in a specific orbit.Luminescence coupling is a special phenomenon appearing between sub-cells within III-V multijunction solar cells,which can be demonstrated by the EQE“artifacts”signals at certain wavelengths.The luminescence coupling is detected and originated from luminous sub-cells with high internal luminescence efficiency,and it was found to be influenced by bias intensities,non-uniform solar cells and configurations.This effect can be described using a circuit model of a multi-junction cell containing a controlled current source,wherein the control factor?is determined by radiative recombination behaviors of carriers in solar cells.Calculation shows that photoluminescence and photon recycling would increase the open circuit voltage and the short-circuit current of multijunction solar cells.After 1×10¹⁵ cm^-22 1 MeV electron irradiation on GaInP/GaAs/Ge cells,carrier recombination rate increases,but the photons emission and photon recycling decrease.It was determined that the controlled factor?of Ge sub-cell decreases from 0.51 to 0.04 with the disappearance of EQE"artifacts"signals.It was found that according to the luminescence coupling mechanisms,it is necessary to introduce a Bragg reflector within the multijunction solar cells and carefully to choose its central wavelength,realizing optimized output current through enhancing the short-circuit current of the upper sub-cells while controlling that of the underneath sub-cells.The photoelectric conversion behavior and luminescence coupling effect of multi-junction cells were found showing significant modulation effects due to changes of incident light parameters.Taking new AlGaInP cell as the object,it is found that the EQE spectra will increase slowly with the increase of light intensity firstly,followed by a linear increase,but finally saturating.This is due to that with increasing the incident light intensity generated excess carriers will fill up the traps gradually up to saturation in solar cells.After 50 keV proton irradiation on AlGaInP cells,the defect concentration increases drastically to decrease the minority carrier diffusion length.It was found also that during the EQE measurement,the bias light could narrowed the space charge region,thus weakens the light modulation effect,or even causes the opposite effect.Based on the above-mentioned light modulation effect,in III-V multi-junction solar cells,one could investigate that the incident light source with different spectra could change significantly the current matching and the power-load relationship between the sub-cells,resulting in different output electrical properties of the multi-junction cell.Luminescence coupling and optical modulation effects in multi-junction cells result in multi-level opto-electric coupling behaviors through carrier transportation and circuit coupling effects.The weaker incident-light conditions show more serious degradation of the solar cells,implying a magnifying effect on the irradiation damage.According to the light modulation behaviors,a new power-matching optimization methodology was proposed between sub-cells for optimized design of multi-junction cells,instead of the conventional spectral matching or lattice matching ones,thus to obtain the promising space multi-junction cells with both the high efficiency and high radiation resistance.Low-energy protons cause non-uniform damage in multi-junction cells.Hence,three kinds of triple-junction GaAs solar cells named by G-,B-and M-were selected to research the non-uniform damage behaviors and mechanisms of the solar cells under low-energy proton irradiation with energies in the range of 50-170 keV.It should be noted that the proton ranges reach different functional regions in the cells,thus causing in damage differences.The results indicate that there show different degradation behaviors of electrical properties of the multi-junction cells as the proton energy is changed.Moreover,these degradation behaviors could not be evaluated equivalently to those in the cells irradiated under high-energy protons or electrons using so-called“equivalent displacement damage dose”methodology.In order to interpret the corresponding phenomena,a hypothesis of the"defects band"concept induced by the non-uniform damage process is proposed and then a simulation model was designed and established using a finite difference analytical method?FDM?.The simulation results reveal that under the non-uniform proton-irradiated damage in the cells,the damage in the emitter mainly causes current degradation,while that in the junction region degrades the open circuit voltage significantly,which are consistent with the experimental results.The reasons for the above-mentioned behaviors were in that non-uniform irradiated defects cause decreasing in carrier lifetimes and also mobilities.It is the most important to be noted that a complex behavior of cascading defects in the junction region of the cell was found as the main mechanism leading to a decrease of the shunt resistance and a significant increase in the ideal factor of solar cell diodes.Based on the photoelectric coupling effects and non-uniform damage mechanisms,new methodologies were established to evaluate the degradation behaviors and to optimize the anti-irradiation property of tri-junction solar cells for the harshest particle radiation environments in MEO.First of all,one could calculate to determine the defect distribution functions in the multi-junction solar cells irradiated by the particle spectra?electrons and protons?underneath the cover glass,using the particle transportation codes and the orbital particle spectra model?such as AE9 and AP9 models?.In the meantime,one can also obtain some equivalent damage parameters such as irradiated damage coefficient of k'?v and mobility degradation rate of K?'from the electrical properties degradation rules.Finally,the degradation models of the electrical properties could be reached of the solar cells with non-uniform irradiated damage from the specific orbit particle irradiation.Correspondingly,a method was proposed to optimize the configurations of multi-junction III-V solar cells served in orbits with harsh wide particle spectral irradiations.Three examples were shown as the optimized solar cell configuration for rigid panel of satellites at 10000 km orbit and that for flexible panel at 20000 km orbit.